It is well-known that a conventional polymeric membrane comprising a dense polymeric layer and a porous polymeric layer is a so-called asymmetrical membrane and exhibits a superior permeability and separating property.
The asymmetrical membrane is prepared by spreading a dope solution containing a film-forming polymeric material on a surface of a substrate and by coagulating the resultant thin layer of the dope solution with a coagulating liquid under conditions adequate for simultaneously forming a dense layer and a porous layer. However, it is very difficult to control the coagulating conditions so as to stably produce an asymmetric membrane having both a dense layer and a porous layer in a single coagulating procedure with a satisfactory degree of reproductivity thereof.
Accordingly, a process for producing a polymeric separating membrane having an excellent separating property with a high degree of reproductivity thereof, has been greatly desired by industry.